U.S. patent number 6,711,911 [Application Number 10/301,042] was granted by the patent office on 2004-03-30 for expansion valve control.
This patent grant is currently assigned to Carrier Corporation. Invention is credited to Michel K. Grabon, Ba-Tung Pham, Philippe Rigal.
United States Patent |
6,711,911 |
Grabon , et al. |
March 30, 2004 |
Expansion valve control
Abstract
An expansion valve is controlled in response to sensing
conditions at the outlet of at least one compressor within a
refrigeration loop in a manner that achieves low suction superheat
operation of the compressor. In particular, a discharge superheat
is computed using data obtained from a specific mathematical model
of the compression process corresponding to the current capacity
stage of the compressor. The position of the expansion valve is
controlled so as to result in an actual discharge superheat being
within a predetermined dead band amount of the computed discharge
superheat.
Inventors: |
Grabon; Michel K. (Bressolles,
FR), Rigal; Philippe (Savigneux, FR), Pham;
Ba-Tung (Chassieu, FR) |
Assignee: |
Carrier Corporation
(Farmington, CT)
|
Family
ID: |
31993664 |
Appl.
No.: |
10/301,042 |
Filed: |
November 21, 2002 |
Current U.S.
Class: |
62/225;
62/175 |
Current CPC
Class: |
F25B
41/31 (20210101); F25B 41/35 (20210101); F25B
2500/19 (20130101); Y02B 30/70 (20130101); F25B
2700/21152 (20130101); F25B 2400/075 (20130101); F25B
2600/2513 (20130101); F25B 2700/1931 (20130101); F25B
2700/1933 (20130101) |
Current International
Class: |
F25B
41/06 (20060101); F25B 041/04 () |
Field of
Search: |
;62/225,208,209,210,175 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Norman; Marc
Claims
What is claimed is:
1. A process for controlling an expansion device within a
refrigerant loop of a cooling system, said process comprising the
steps of: sensing temperature and pressure at the outlet of at
least one compressor within the refrigerant loop; obtaining a
saturated discharge temperature based upon the sensed pressure at
the outlet of the at least one compressor; computing a discharge
superheat at the outlet of the at least one compressor using the
saturated discharge temperature; and controlling the expansion
device within the refrigerant loop in response to the computed
discharge superheat, wherein said step of computing a discharge
superheat includes the step of generating a mathematical algorithm
for computing the discharge superheat that is based upon the
capacity of the at least one compressor within the refrigerant
loop.
2. The process of claim 1 wherein said step of computing a
discharge superheat includes tho steps of: sensing the pressure
between the evaporator and the inlet of the at least one
compressor; and computing a theoretical discharge temperature
corresponding to a zero degree suction superheat as a function of
the sensed pressure at the outlet of the at least one compressor
and the sensed pressure between the evaporator and the inlet of the
at least one compressor.
3. The process of claim 2 wherein said step of computing a
theoretical discharge temperature includes using at least one
constant applied to the sensed pressure at the outlet of the at
least one compressor or the sensed pressure between the evaporator
and the inlet of the at least one compressor wherein the constant
is selected based upon the capacity of the at least one compressor
within the refrigerant loop.
4. The process of claim 2 wherein said step of computing a
discharge superheat includes the steps of: computing a theoretical
discharge superheat based upon the computed theoretical discharge
temperature corresponding to zero degree suction heat; and adding a
discharge superheat correction factor to the computed discharge
superheat.
5. The process of claim 2 wherein the refrigerant loop contains a
plurality of compressors each of which may be activated in response
to the cooling demand placed upon the cooling system and wherein
said step of computing a discharge superheat using the saturated
discharge temperature comprises the step of: generating a
mathematical algorithm for computing the discharge superheat that
is based upon the number of active compressors within the
refrigerant loop.
6. The process of claim 2 wherein the refrigerant loop contains a
plurality of compressors each of which may be activated in response
to the cooling demand placed upon the cooling system and wherein
said step of sensing temperature and pressure of the outlet of the
at least one compressor includes the step of sensing temperature
and pressure at a common manifold outlet of the compressors and
wherein said step of computing a discharge superheat includes the
steps of: sensing pressure between an evaporator and a common
manifold inlet of the compressors; and computing at least one
theoretical discharge temperature corresponding to a zero degree
suction superheat as a function of the sensed pressure at the
common manifold outlet of the compressors and the sensed pressure
at the common manifold inlet of the compressors.
7. The process of claim 6 wherein said step of computing at least
one theoretical discharge temperature includes using at least one
constant applied to the discharge pressure or the suction pressure
wherein the constant is selected based upon the number of activated
compressors within the refrigerant loop.
8. The process of claim 6 wherein said step of computing a
discharge superheat includes the steps of: computing a theoretical
discharge superheat based upon the computed theoretical discharge
temperature corresponding to zero degree suction heat; and adding a
discharge superheat correction factor to the computed discharge
superheat.
9. The process of claim 1 wherein said of controlling the expansion
device in response to the computed discharge superheat includes the
steps of: determining an actual discharge superheat; determining
whether the actual discharge superheat is within a range of
predetermined values; and changing the refrigerant flow rate
through the expansion device when the actual discharge superheat is
outside of the range of predetermined values.
10. The process of claim 9 defining the predetermined values by
using at least one predetermined variance with respect to the
computed discharge superheat.
11. The process of claim 1 wherein said step of obtaining a
saturated discharge temperature comprises the step of: obtaining a
saturated discharge temperature for a particular capacity of the at
least one compressor.
12. The process of claim 1 further comprising the step of: noting
the current capacity for the at least one compressor within the
refrigerant loop; and using the noted capacity to determine one or
more values used in said step computing the discharge
superheat.
13. A system for controlling an expansion device within a
refrigerant loop of a cooling system, said system comprising: a
sensor for sensing temperature at the outlet of at least one
compressor within the refrigerant loop; a sensor for sensing
pressure at the outlet of the at least one compressor within the
refrigerant loop; and a processor operative to obtain a saturated
discharge temperature based upon the sensed pressure at the outlet
of the at least one compressor, said processor being operative to
commute a discharge superheat using the saturated discharge
temperature, said processor being furthermore operative to control
the expansion device within the refrigerant loop in response to the
computed discharge superheat, wherein said processor is furthermore
operative to generate a mathematical algorithm when computing the
discharge superheat, the algorithm being based upon the current
capacity of the at least compressor within the refrigerant
loop.
14. The system of claim 13 furthermore comprising: a sensor for
sensing pressure between the outlet of the evaporator and the inlet
of the at least one compressor in the refrigerant loop; and wherein
said processor is operative when computing a discharge superheat to
compute a theoretical discharge temperature corresponding to a zero
degree suction superheat as a function of the sensed pressure at
the outlet of the at least one compressor and the sensed pressure
between the evaporator and the inlet of the at least one
compressor.
15. The system of claim 14 wherein said processor uses when
computing a theoretical discharge temperature at least one constant
applied to the sensed pressure at the outlet of the at least one
compressor or the sensed pressure between the evaporator and the
inlet of the at least one compressor wherein the constant is
selected based upon the current capacity of the at least one
compressor within the refrigerant loop.
16. The system of claim 14 wherein said processor is operative when
computing a discharge superheat to compute a theoretical discharge
superheat based upon the computed theoretical discharge temperature
corresponding to zero degree suction heat; and to add a discharge
superheat correction factor to the computed discharge
superheat.
17. The system of claim 13 wherein the refrigerant loop contains a
plurality of compressors each of which may be activated in response
to the cooling demand placed upon the cooling system and wherein
said processor is operative when computing a discharge superheat to
generate a mathematical algorithm for computing the discharge
superheat that is based upon the number of active compressors
within the refrigerant loop.
18. The system of claim 13 wherein the refrigerant loop contains a
plurality of compressors each of which may be activated in response
to the cooling demand placed upon the cooling system and wherein
said processor is operative when sensing temperature and pressure
at the outlet of the at least one compressor to sense temperature
and pressure at a common manifold outlet of the compressors and
wherein said processor is furthermore operative when computing a
discharge superheat to compute a theoretical discharge temperature
correspond to a zero degree suction superheat as a function of the
sensed pressure at the common manifold outlet of the
compressors.
19. The system of claim 18 wherein said processor is operative when
computing a theoretical discharge temperature to use at least one
constant applied to the discharge pressure wherein the constant is
selected based upon the number of activated compressors within the
refrigerant loop.
20. The system of claim 18 wherein said processor is operative when
computing a discharge superheat to compute a theoretical discharge
superheat based upon the computed theoretical discharge temperature
corresponding to zero degree suction heat and wherein said
processor is furthermore operative to add discharge superheat
correction factor to the computed discharge superheat.
21. The system of claim 13 wherein said processor is furthermore
operative to compute an actual discharge superheat and determine
whether the actual discharge superheat is within a range of
predetermined values when controlling the expansion device and to
change the refrigerant flow rate through the expansion device when
the actual discharge superheat is outside of the range of
predetermined values.
22. The system of claim 21 wherein said processor is furthermore
operative to define the predetermined values by using at least one
predetermined variance with respect to a computed discharge
superheat.
23. The system of claim 13 wherein said processor is operative to
obtain a saturated discharge temperature for a particular capacity
when obtaining a saturated discharge temperature.
24. The system of claim 13 wherein said processor is furthermore
operative to note the current capacity for the at least one
compressor within the refrigerant loop and to thereafter use the
noted current capacity to determine one or more values used in
computing the discharge superheat.
Description
BACKGROUND OF THE INVENTION
This invention relates to expansion devices used in refrigeration
and air conditioning systems to adjust the flow of refrigerant in a
refrigeration circuit. In particular, this invention relates to
expansion devices used in refrigeration and air conditioning
systems that require several stages of cooling capacity.
A role of an expansion device in refrigeration and air conditioning
systems requiring several stages of cooling capacity is to
configure its geometry (orifice size) in such a way that the
refrigerant mass flow through the device corresponds exactly to the
mass flow generated by the one or more compressors. This control of
refrigerant flow must also maintain an optimum gas condition of the
refrigerant entering the suction side of the compressor.
Thermal expansion valves, TXVs, and electronically controlled
expansion valves, EXVs, are used in refrigeration and air
conditioning systems. The traditional approach for controlling TXVs
or EXVs is to provide a signal that opens or closes the valve based
on an evaluation of suction gas superheat. Superheat is the
difference between actual refrigerant temperature and saturated
refrigerant temperature (temperature corresponding to the phase
change). In thermal expansion valves (TXV) the type of control used
is analog. The TXV is equipped with a bubble in a compressor
suction line which senses the refrigerant temperature. A pressure
signal corresponding to the suction line pressure is provided as
well. Based on these two signals (refrigerant temperature and
refrigerant pressure at the compressor inlet), the analog system
adjusts the TXV opening to maintain a requested level of suction
superheat (set point). This kind of expansion device has a limited
range of application. If the refrigeration circuit can operate with
a large span of capacities and with a large span of operating
conditions, then the TXV type of controls cannot be optimized in
all possible operating envelopes.
Electronic expansion devices (EXV) are usually electronically
driven valves that are adjusted based on more or less sophisticated
control algorithms. The adjusted EXV opening should be such that
the refrigerant entering the evaporator fully evaporates in the
evaporator. In this regard, there should preferably be no liquid
refrigerant droplets leaving the evaporator. This is extremely
important because excessive amounts of liquid refrigerant entering
the compressor from the evaporator may result in compressor
failure. To be sure that no liquid refrigerant leaves the
evaporator, significant suction superheat is usually required. This
requirement to optimize evaporator effectiveness counters the
objective of achieving the best system efficiency by minimizing the
suction superheat requirement.
To satisfy a safe operation of the compressor and also achieve good
overall system efficiency, the suction superheat is usually
maintained at a level of approximately 5.degree. C. Significant
improvement of system efficiency would be obtained if one could
however guarantee that no liquid refrigerant droplets enter the
compressor with a lower suction superheat. It is however extremely
difficult to measure the temperature difference defining suction
superheat at a magnitude lower than 5.degree. C. with reasonable
confidence. In particular when the refrigerant is close to
saturation, problems of refrigerant misdistribution or refrigerant
homogeneity makes it almost impossible to measure this temperature
difference.
SUMMARY OF THE INVENTION
The invention provides for the control of an expansion valve
without relying on measuring temperature at the suction side of a
compressor. In particular, the control of the expansion valve is
premised on a computation of discharge superheat using a
mathematical algorithm based upon the current capacity of one or
more activated compressors. The computation of the discharge
superheat is preferably based on sensed suction and discharge
pressures for the one or more compressors. The computed discharge
superheat is compared with an actual discharge superheat that is
based on a sensed discharge gas temperature. The comparison
preferably permits the actual discharge superheat to be within a
prescribed amount of the computed discharge superheat. This
computational process has a much lower likelihood of error when
contrasted with a computation based on sensing suction temperature.
In this regard, when the compressor or compressors operate in the
so called "flooded condition" (no suction superheat), the
measurement of conditions of the refrigerant in an evaporator
leaving section or compressor entering section gives no idea about
the refrigerant quality (quantity of liquid refrigerant in a
mixture) entering the compressor. In reality, when the refrigerant
entering the compressor is a saturated gas or mixture of the
saturated gas and liquid, the refrigerant temperature is equal to
refrigerant saturated temperature with suction superheat being
equal to 0. It is impossible to make a distinction between
acceptable, transient operation with some liquid droplets entering
the compressor and an operation with large amount of liquid, which
results in a very rapid compressor failure.
Computing superheat based on the conditions of the refrigerant at
discharge from the compressor allows a control to clearly
distinguish refrigerant quality (amount of liquid in a mixture)
entering the compressor. Knowing the refrigerant quality while
operating with minimal or no suction gas superheat allows for an
appropriate control of the EXV opening in a transient, low suction
superheat.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the present invention, reference
should now be made to the following detailed description thereof
taken in conjunction with the accompanying drawings wherein:
FIG. 1 is a schematic view of a chiller system for delivering
chilled water to a downstream load;
FIG. 2 is a graphical depiction of the compression of refrigerant
vapor by the compressor operating at a particular capacity within
the chiller system of FIG. 1;
FIG. 3 is an enlargement of a portion of FIG. 2 depicting certain
variables having values that are either stipulated or computed by a
controller associated with the system of FIG. 1;
FIG. 4 is a flow chart of a method used by a controller associated
with the chiller system of FIG. 1 to control the expansion device
within the refrigerant loop of the chiller based on certain of the
variables in FIG. 3
FIG. 5 is a schematic view of an alternative chiller system having
parallel compressors; and
FIG. 6 is a flow chart of a method used by a controller associated
with the chiller system of FIG. 5 to control the expansion device
within the refrigerant loop of the chiller.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, a chiller system 10 delivers chilled water via
a pipeline 12 to various distribution points that are not shown. It
is to be appreciated that the distribution points may be one or
more fan coil heat exchangers that condition air flowing through
the fan coil heat exchangers having a heat exchange relationship
with the chilled water. The resulting conditioned air is provided
to spaces to be cooled. It is finally to be noted that the water
circulating through each fan coil heat exchanger is ultimately
pumped back into the chiller 10 by a water pump 14.
The chiller 10 is seen to include a condenser 16 having a fan 18
associated therewith. The heat of condensation of the hot
refrigerant vapor refrigerant passing through the condenser 16 is
removed by the flow of air produced by the fan 18. This produces
high-pressure sub cooled liquid refrigerant at the outlet end of
the condenser 16. This high-pressure sub cooled liquid refrigerant
flows into an expansion valve 20 and is discharged at a lower
pressure. The refrigerant thereafter enters an evaporator 22. The
liquid refrigerant in the evaporator will extract heat from water
circulating in one or more pipes immersed in the liquid refrigerant
within the evaporator. The circulating water in the one or more
pipes in the evaporator is the water that has been returned from
the distribution points via the pump 14. The resulting chilled
water leaves the evaporator 22 and is returned to the distribution
points via the pipeline 12. On the other hand, low-pressure
refrigerant vapor from the evaporator is directed to the suction
inlet of a compressor 24. The compressor 24 compresses the
refrigerant vapor that is thereafter discharged to the condenser
16. The compressor 24 preferably includes at least two stages of
compression that may be sequentially activated so as to meet the
cooling demands placed upon the chiller 10. In this regard, the
single compressor 24 of FIG. 1 may for example be a reciprocating
compressor having up to six cylinders in which two, four or six
pistons could be activated depending on the cooling requirements
placed on the system.
Cooling demands on this system are typically based on sensing the
temperature of the water leaving the chiller and comparing the same
with a set point temperature for the chilled water. For example, if
the set point temperature is 7.degree. C. then the chiller controls
will define a cooling capacity that will normally achieve a chilled
water temperature of 7.degree. C. for the water leaving the
chiller. If the leaving water temperature is higher than 7.degree.
C., then the chiller controls will add additional cooling capacity
by activating additional pistons. If leaving water temperature is
lower than 7.degree. C., then the cooling capacity is higher than
needed and the chiller controls will reduce cooling capacity by
cutting back on the number of activated pistons.
Referring again to the compressor 24, a discharge pressure sensor
26 and a reference temperature sensor 28 are positioned at the
outlet of the compressor. A suction pressure sensor 30 is
positioned between the outlet of the evaporator 22 and the inlet of
the compressor 24. The outputs of the sensors 26, 28, and 30 are
connected to a controller 32. As will be explained in detail
hereinafter, the controller 32 is operative to control a motor 34
associated with the expansion valve 20 so as to open or close the
expansion valve and thereby control the mass flow of refrigerant to
the evaporator 22. The control is accomplished in a manner that
permits the suction superheat to be minimized at the inlet of the
compressor 24 while maintaining an adequate refrigerant vapor
status so as to not introduce harmful refrigerant liquid droplets
into the compressor.
Referring to FIG. 2, a vapor compression curve is illustrated for a
particular compressor capacity of the compressor 24. It is to be
appreciated that the curve will define a saturated suction
temperature, "SST", for a given suction pressure, "SP", sensed by
the sensor 30. It is also to be appreciated that the curve will
define a saturated discharge temperature, "SDT", for a given
discharge pressure, "DP", sensed by the sensor 26.
Referring now to FIG. 3, an enlargement of a portion of the vapor
compression curve of FIG. 2 is further illustrated in conjunction
with two sloped lines that define certain variables that are to be
computed by the controller 32. In particular, a sloped dashed line
SL.sub.theo is preferably tangent with the vapor compression curve
at a point defined by SST and DP. The dashed line will hence
generally represent the slope of the vapor compression curve at
this point. This point in FIG. 3 will be hereinafter referred to as
zero suction superheat which means that there is zero degrees in
temperature of superheat above the saturated suction temperature
SST. The sloped line SL.sub.theo intersects the discharge pressure
line DP at a point defined as T.sub.theo.sub..sub.-- .sub.dis which
is defined as the theoretical discharge temperature that would be
experienced at the sensor 28 for a zero suction superheat. The
difference between the T.sub.theo.sub..sub.-- .sub.dis and the
saturated discharge temperature SDT is the theoretical discharge
superheat DSH.sub.theo. As will be explained hereinafter, an
optimum discharge superheat DSH.sub.opt is preferably computed by
adding a discharge superheat correction factor DSH.sub.cf to the
theoretical discharge superheat DSH.sub.theo. A sloped line
SL.sub.opt drawn parallel to the sloped line SL.sub.theo intersects
the suction pressure line SP so as to define what would be an
optimum suction superheat SSH.sub.opt corresponding to the computed
DSH.sub.opt.
Referring now to FIG. 4, a process utilized by a programmable
processor within the controller 32 is illustrated. The process
begins with a step 40 wherein the programmed processor accesses the
currently defined refrigeration circuit capacity. As has been
previously discussed, the refrigeration circuit capacity will
depend on the cooling demands placed on the system of FIG. 1. The
chiller controls will selectively activate the appropriate number
of stages of compression to meet these cooling demands. For
instance, if the compressor has six cylinders that may be activated
in successive pairs, then the number of so activated cylinders will
be noted in step 40.
The processor proceeds to a step 42 and either directly reads
values or indirectly reads previously stored values of sensed
discharge pressure from sensor 28, sensed discharge temperature
from sensor 26 and a sensed suction pressure from sensor 30. These
read values are stored as "DP", "T.sub.dis ", and "SP"
respectively. The processor proceeds in a step 44 to compute or
otherwise obtain a value for saturated discharge temperature,
"SDT", based upon the value of "DP". As has been noted previously
with respect to FIG. 2, a mathematical model of compression can be
used to obtain "SDT". The processor next proceeds to a step 46 and
computes an actual discharge superheat, "DSH.sub.act ", by
subtracting "SDT" from "T.sub.dis ". The processor thereafter
proceeds in a step 48 to read certain previously stored
configurable constants for discharge superheat correction factor
"DSH.sub.cf " and a permissible discharge superheat deadband
"DB".
The processor proceeds in a step 50 to read a set of coefficients
for the defined capacity of step 40 that will thereafter be used in
a computation carried out in a step 52. It is to be understood that
step 50 is preferably implemented by accessing a stored set of
coefficients that have been developed from a vapor compression
curve such as shown in FIG. 2 for the defined capacity. The
processor proceeds to a step 52 and computes an optimal compressor
discharge superheat, "DSH.sub.opt ". The algorithm used to compute
this optimal compressor discharge superheat may be computed in one
or more separate steps. In the preferred embodiment, a theoretical
discharge temperature, T.sub.theo.sub..sub.-- .sub.dis, is a
discharge temperature corresponding to 0.degree. C. suction
superheat. It is first calculated based on being a function of
suction pressure, "SP", discharge pressure,"DP" and a given value
of saturated discharge temperature, "SDT". This may be expressed as
follows:
Suction pressure "SP" and Discharge pressure "DP" are sensed
values. Saturated discharge temperature, "SDT", may be either
obtained or calculated for a sensed discharge pressure of the
compressor operating at a given compressor capacity. A.sub.i, is a
constant and B.sub.i, C.sub.i and D.sub.i are coefficients for a
given compressor capacity, indicated by the subscript "i". Values
of Ai, B.sub.i, C.sub.i and D.sub.i define a linear relationship
between T.sub.theo.sub..sub.-- .sub.dis and SP, DP, and SDT. This
linear relationship is indicated by the sloped line SL.sub.theo in
FIG. 3. It is to be appreciated that this linear relationship can
be generated using appropriate mathematical modeling principles for
vapor compression at a given capacity of compression within a given
refrigeration circuit. It is also to be appreciated that values of
A.sub.i, B.sub.i, C.sub.i and D.sub.i can be generated for the
three specific compressor capacities for the compressor 24 of FIG.
1. In this case, the programmed processor within the controller
will have access to the following sets of coefficients:
Compressor_capacity_1: A.sub.1, B.sub.1, C.sub.1, D.sub.1
Compressor_capacity_2: A.sub.2, B.sub.2, C.sub.2, D.sub.3
Compressor_capacity_3: A.sub.3, B.sub.3, C.sub.3, D.sub.3
It is to be appreciated that the above mathematical algorithm used
to compute T.sub.theo.sub..sub.-- .sub.dis can also be based on
system variable measurements other than "SP", and "DP". For
example, it is possible to build a mathematical algorithm to
calculate a theoretical discharge temperature based on measured
compressor current, compressor input power or cooling capacity
mixed with a measurement of saturated refrigerant temperature
measured directly in the condenser and the cooler and can have a
different number of constants and coefficients.
Once T.sub.theo.sub..sub.-- .sub.dis is computed, then a
theoretical discharge superheat, DSH.sub.theo, can be computed as
follows:
An optimum discharge superheat DSHopt is preferably calculated by
adding the discharge superheat correction factor "DSH.sub.cf "
obtained in step 48 to DSH.sub.theo as follows:
DSH.sub.cf is depicted in FIG. 3 as a constant to be added to
discharge superheat. This corresponds to a permissible amount of
suction superheat SSH defined by the sloped line SL.sub.opt. In
theory the best effectiveness of a system is achieved when suction
superheat SSH is equal to 0.degree. C. In reality, operating with a
suction superheat SSH of 1 to 3.degree. C. provides additional
safety for compressor operation while not significantly impacting
system efficiency. The particular value of DSH.sub.cf is chosen so
as to correspond to an SSH of 1 to 3.degree. C. for a given
compressor capacity within the refrigeration circuit.
The processor proceeds to a step 54 a nd inquires as to whether
DSH.sub.act computed in step 46 is less than DSH.sub.opt computed
in step 52 minus the permissible discharge superheat deadband "DB".
DB is used to prevent instability in the control of the expansion
device 20. In this regard, expansion devices have their own
"resolution". For example, the valve position of an expansion
device may vary 1%. This 1% variation will usually correspond to a
1% variation in the opening of the device. This will in turn result
in an increase or decrease of refrigerant flow entering the
evaporator which will in turn affect the compressor suction
superheat and eventually discharge superheat. It is hence important
to define a value of DB that is greater that the corresponding
resolution of the valve or opening of the expansion device. For
example, if DB is one half degree Centigrade, then the valve
position or opening of the expansion device 20 will not change if
DSH.sub.act is within 0.5.degree. C. of DSH.sub.opt.
Referring again to step 54, in the event that the answer is no, the
processor proceeds to a step 56 and inquires as to whether
DSH.sub.act is greater than DSH.sub.opt plus DB. If the answer is
again no, then the processor proceeds to an exit step 58.
Referring again to steps 54 and 56, if the answer is yes to either
of these queries, then the processor proceeds to a step 60 and
adjusts the position of the expansion valve 20 through appropriate
signals to the motor 34 so as to satisfy DSH.sub.opt. The processor
thereafter proceeds to exit step 58.
It is to be appreciated that the processor will repeatedly
implement steps 40 though 60 in a timely manner so as to maintain
control of the motor 34 associated with the expansion valve 20. The
amount of time between successive implementations will depend on
the particular motor and associated expansion valve as well as the
refrigerant loop in which the expansion valve operates.
Referring now to FIG. 5, wherein the single compressor
configuration of the chiller system in FIG. 1 has been replaced
with three compressors 24-1, 24-2, and 24-3 operating in parallel.
It is to be appreciated that controls for the chiller will add or
subtract cooling capacity by adding or subtracting one or more of
the compressors operating in parallel. If each compressor is
identical then each compressor that is added or subtracted will
produce the same discharge temperature and each will have the same
compression process model such as shown in FIG. 2. On the other
hand, if the compressors are different (different compressor
effectiveness) then each compressor discharge temperature may be
different and it may be necessary to calculate or obtain discharge
temperature corresponding to each different compressor based on
specific models for each compressor. It is to be noted that the
pressure sensor 26 and the temperature sensor 28 are each located
in a common discharge manifold for the compressors 24-1, 24-2, and
24-3. It is also to be noted that the pressure sensor 30 is located
in a common input manifold to the identical compressors 24-1, 24-2,
and 24-3.
Referring now to FIG. 6, a process utilized by a programmable
processor within the controller 32 is illustrated for the chiller
configuration of FIG. 4. It is to be noted that most of the steps
in FIG. 5 are the same as those in the process of FIG. 3. In this
regard, current refrigerant capacity is read in a step 62. Since
the compressor configuration of FIG. 4 is three parallel
compressors, the processor will note how many of these compressors
have been activated. The sensors 26, 28, and 30 for the system of
FIG. 4 are read in step 64 before computing a saturated discharge
temperature "SDT" in step 66 based upon the value of DP read from
the pressure sensor 26. In this regard, the saturated discharge
temperature is preferably based on the compression process model
for the number of activated compressors indicated by the
refrigeration circuit capacity noted in step 62.
Referring now to step 68, the processor calculates an actual
discharge superheat, DSH.sub.act based on the read discharge
temperature from sensor 26 and "SDT" as computed in step 66. The
processor now proceeds in a step 70 and reads the configurable
constants DSH.sub.cf and DB. The processor proceeds in step 72 to
compute an optimal compressor discharge superheat "DSH.sub.opt (i)"
for each activated compressor. This is preferably accomplished by
first computing a theoretical discharge temperature for each
compressor as follows:
Suction pressure (SP) and Discharge pressure (DP) are sensed
values. Saturated discharge temperature (SDT) is either obtained or
calculated for the sensed discharge pressure in step 66. A.sub.i is
a constant and B.sub.i, C.sub.i and D.sub.i are coefficients
corresponding to the specific compression capacity of the given
compressor. Values for A.sub.i, B.sub.i, C.sub.i and D.sub.i will
have been previously derived and stored for use in the computation.
If the three compressors each have their own particular capacities,
then the programmed processor within the controller will have
access to the following sets of coefficients:
Compressor_24-1: A.sub.1, B.sub.1, C.sub.1, D.sub.1
Compressor_24-2: A.sub.2, B.sub.2, C.sub.2, D.sub.3
Compressor_24-3: A.sub.3, B.sub.3, C.sub.3, D.sub.3
It is to be appreciated that if each of the compressors are the
same, then the programmed processor will only need to perform one
computation of Ttheo.sub..sub.-- .sub.dis since the values of
A.sub.i, B.sub.i, C.sub.i and D.sub.i will be the same.
Once T.sub.theo.sub..sub.-- .sub.dis (i) is computed for each
active compressor, then a theoretical discharge superheat,
DSH.sub.theo (i) for each active compressor can also be computed as
follows:
An optimum discharge superheat for each compressor is next
preferably calculated by adding the discharge superheat correction
factor "DSH.sub.cf " obtained in step 70 to DSH.sub.theo (i) for
each compressor as follows
The processor proceeds in a step 74 to select the minimum
DSH.sub.opt (i) computed in step 72 and sets the same equal to
DSH.sub.opt. The processor now proceeds to step 76 and inquires as
to whether DSH.sub.act computed in step 68 is less than DSH.sub.opt
computed in step 74 minus the permissible discharge superheat
deadband "DB". In the event that the answer is no, the processor
proceeds to a step 78 and inquires as to whether DSH.sub.act is
greater than DSH.sub.opt plus DB. If the answer is again no, then
the processor proceeds to an exit step 80.
Referring again to steps 76 and 78, if the answer is yes to either
of these queries, then the processor proceeds to a step 82 and
adjusts the position of the expansion valve 20 through appropriate
signals to the motor 34 so as to satisfy DSH.sub.opt. The processor
thereafter proceeds to exit step 80.
It is to be appreciated that the processor will repeatedly
implement steps 62 though 82 in a timely manner so as to maintain
control of the motor 34 associated with the expansion valve 20. The
amount of time between successive implementations will depend on
the particular motor and associated expansion valve as well as the
refrigerant loop in which the expansion valve operates.
It is to be appreciated that a preferred embodiment of the
invention has been disclosed. Alterations or modifications may
occur to one of ordinary skill in the art. For instance, the
chiller systems of FIG. 1 or 5 could be replaced with almost any
type of air conditioning or refrigeration system employing an
electronically controlled expansion device to be controlled using
the processes of FIG. 4 or the process of FIG. 6. Furthermore, the
processes of FIG. 4 or 6 could be modified so as to automatically
repeat after a predefined time through an appropriate delay being
implemented instead of the exit step.
It will be appreciated by those skilled in the art that further
changes could be made to the above-described invention without
departing from the scope of the invention. Accordingly, the
foregoing description is by way of example only and the invention
is to be limited only by the following claims and equivalents
thereto.
* * * * *